Re: [PATCH] nldbl-compat.c: Include math.h before nldbl-compat.h.

[Joseph Myers <joseph at codesourcery dot com>]

On Sun, 11 Mar 2018, Zack Weinberg wrote:

> Jeff Law noticed that native PowerPC builds were broken by my having
> made math_ldbl_opt.h not include math.h.  nldbl-compat.c formerly got
> math.h via libioP.h and math_ldbl_opt.h, *without* __NO_LONG_DOUBLE_MATH;
> after my change it got it via nldbl-compat.h *with* __NO_LONG_DOUBLE_MATH,
> but __NO_LONG_DOUBLE_MATH mode is forbidden on hosts that define
> __HAVE_DISTINCT_FLOAT128, so the build breaks.  I don't know why this
> didn't come up in a build-many-glibcs cycle.

It did, with GCC mainline (older GCC doesn't default to supporting 
_Float128 on powerpc64le but only supports it when certain options are 
passed).

https://sourceware.org/ml/libc-testresults/2018-q1/msg00447.html

sysdeps/powerpc/bits/floatn.h makes sure not to enable __HAVE_FLOAT128 and 
__HAVE_DISTINCT_FLOAT128 in the __NO_LONG_DOUBLE_MATH case, so you should 
only have a problem when nldbl-compat.h is included late (after 
bits/floatn.h is included) but other headers disliking that combination 
are included even later.

> I pushed this as a quick fix, but there's a deeper problem: presumably
> nldbl-compat.h defines __NO_LONG_DOUBLE_MATH for a reason, but if we

To disable long double function declarations, so that e.g. nldbl-sin.c can 
define sinl with the "wrong" type (but the type correct for the ABI for 
which it's being built).

The files such as nldbl-sin.c are for libnldbl_nonshared.a, which 
supposedly supports -mlong-double-64 with compilers not supporting asm 
redirection (if asm redirection is supported, it's used so that calls to 
sinl etc. call sin etc. in that case).  I suspect in fact this case hasn't 
worked for a long time - see 
<https://sourceware.org/ml/libc-alpha/2014-08/msg00497.html> with an 
incorrect patch attempting to fix a build failure with such a compiler but 
without actually resulting in correct declarations.  So there may be a 
case for eliminating libnldbl_nonshared.a, in which case this special 
__NO_LONG_DOUBLE_MATH override might not be needed (and it would become a 
more normal macro, only ever defined by bits/long-double.h).

-- 
Joseph S. Myers
joseph@codesourcery.com